M.Sc-1st Year-PHYSICAL CHEMISTRY-AS-2145

(Dr. K.V.S.Ranganath)

1. What is an oscillatory reaction: The oscillatory reaction is the variation of concentration of

intermediates with time. It is very difficult to find one reaction, which goes reversible by itself. Auto

catalysis is the key step in the oscillatory reactions. The most important characteristic of oscillatory

reaction is steady state approximation is not applicable.

Example: Oxidation of malonic acid by Ce3+

/Ce4+

in the presence of H2SO4

2. Physical Interpretion of Bjerrum’s critical distance: The concept of association of ions to form ion

pairs was studied by Bjerrum. The probability of finding two oppositely charged ions together is a

minimum, can be obtained

r min= z+z-ϵ2/ 2DkT. According to Bjjerum, all ions lying within a sphere of radius r should be regarded

as associated to form ion-pair, where as those outside this sphere may be considered to be free. The

higher value of “r min,” the greater the probability of the occurrence of ion-pairs.

3. According to Debye-Huckel theory, oppositely charged ions attract one another. As a result anions,

are more likely to be found near cations in solution and vice-versa. Finally, over period of the time, it

leads to the formation of ionic atmosphere. The energy, and therefore the chemical potential of any

given central ion are lowered as a result of electrostatic attraction with its ionic atmosphere. This

model leads to the result that at very low concentrations the activity coefficient can be calculated from

Debye-Huckel Limiting Law:

log = -(z+z-)AI1/2

, where A= 0.509 and “I” is the dimensionless ionic strength of the solution. This

indicates that log of activity coefficient must decrease linearly with square root of ionic strength. At

infinite dilution, where the interionic forces are negligible; I is 0 and tends to 1.

It gives the concept of ion pair formation; charging process; asymmetric and electrophoteric effects

explained by D-H theory. The Debye-Huckel Onsagar equation is: log = -(z+z-)AI1/2

/ (1+ BI1/2

).

Where “a” is the minimum distance to form

ion pair and “q” is the maximum distance to

form ion pair. Force of attraction is strong if

ions are at a distance of “r”. The probability

of formation of ion pair according to Bjjerum

is given by 4Cπλ3 ∫e

yy

-4dy

4. Simultaneous reaction:

The general representation of simultaneous reaction is given by:

A X Z

EX; 2Fe + 3CO2 Fe2O3 + 3 CO Fe3O4 + CO2

5. Activity Coefficient:

The activity coefficient (i) measures the degree of departure of substance behavior from ideal or

ideally dilute behavior. It is the ratio of ai/xi; where ai is the activity and xi is the mole fraction. In

ideal solution, the activity coefficient is one. (i = aixi); i is equal to one for ideal.

6. Bimolecular Surface Reactions:

(i) Ethylene hydrogenation over platinum surfaces

(ii) NH3 synthesis catalyzed by iron

(iii) Oxidation of CO on transition metals.

(iv) Cyclohexene hydrogenation on Pt surface.

7. Organic decomposition:

The breakdown of organic raw material into the final product of compositing is known as organic

decomposition. In organic decomposition, kinetics and mainly elementary processes believed to be

playing a major part.

Ex: Pyrolysis of ethane. (main products are ethylene and hydrogen), but significant amounts of

methane and butane are also formed.

8. Sticking probability:

The rate at which a surface is covered by adsorbate depends on the ability of the substrate to

dissipate the energy of incoming particle as thermal motion as it crashes on to the surface. If the

energy is not dissipated quickly, the particle migrates over the surface until a vibration expels it

into the overlying gas or it reaches an edge. The portion of collision with the surface that

successfully lead adsorption is called sticking probability. It is given by s=

Rate of adsorption of particles by the surface

Rate of collision of particles with the surface before adsorption

9. Significance of excessive function:

It is thermodynamic properties of a solution of two liquids are often expressed in terms of

excess function. The excess entropy SE of a mixture is defined as the difference between the

actual Entropy of the solution and the entropy of a hypothetical ideal solution with the same

Temp, Pressure and composition: SE = S-S

id. Similarly we can also define excess functions of

other parameters like enthalpy, volume, Gibbs free energy.

10. The decomposition of PH3 on tungsten is First order at low pressures and zero-order

at high pressures

Long Answer:

2. Consider 1 mole of an electrolyte dissociating into +

cations and - anions

G = ns s + n = ns s + n+ + + n- -

We know to define the activities: (“+” is for cation and “–“ is for anion)

= + RT ln a; + = + + RT ln a+

- = - + RT ln a- ; = + RT ln a

Relationship between a and a: (activity and mean activity)

We know: = / ; where is the valency.

= + RT ln a = ( + RT ln a). The relation between activity and mean activity of

electrolyte is (a)= a

Activity Coefficient varies with concentration of a solution:

The Debye-Huckel limiting law shows that the activity coefficient is related to the ionic

strength, I in the following way:

ln = - A z+ z- I ½

where A is a constant (= 1.172 at 298 K), z+ z- the valence factor and I is the ionic strength

which is define by the equation:

I = ½ zi2mi

where mi is the concentration of the ith ion concentration. The summation, , is taken over all

the possible ions in the solution.

For very concentrated solutions, using concentration based on weight of solvent may offer a

better approximation than using concentration based on volume. However, at this level, we

are only introducing the concept of ionic strength for the calculation of the activity

coefficient.

Equivalent conductivity is the conductance of volume of a solution containing one equivalent

mass of a dissolved substance when placed between two parallel electrodes; which are at

distance of 1 cm apart. It increases on dilution because of decrease in conductivity is more

compensated with increase in value of (1/c) on dilution.

When association occurs; At higher concentration; a non conducting pair is formed and this

reduces equivalent conductance. Ion pair is more predominant in low dielectric solvent

media. Conductance decreases with increase in conc. and at low dielectric constant

3. Fouss model

4. Composite Reactions: Composite reactions are consecutive, simultaneous and

opposing reactions

4bKinetic Chain Length:

Chain length is defined as the ratio of rate of formation of product to the rate of initial radical

formation

= Rp / Ri (OR)

Number of propagation steps per initiation step (OR)

Ratio of the rate of propagation step to the rate of initiation

Chain length () = propagation/ Initiation = propagation/ termination

5. (a) The adsorption of a gas is described by the Langmuir isotherm with K = 0.75 kPa-1

at

25C. Calculate the pressure at which the fractional surface coverage is (i) 0.15 and

(ii) 0.95

The Langmuir adsorption isotherm is given by = Kp / 1+ Kp

“” is the fraction of the surface covered; K is the equilibrium constant; ‘p’ is the pressure of

the gas.

From the above equation, P = / (1-)K; P = 0.15/(1-0.15)* 0.75;

P = 0.15/0.64 = 0.23 Pa.

P = 0.95/ (1-0.95)*0.75 = 0.95/0.38 = 2.5 Pa.

(b) If “I” is the inhibitor on a unimolecular surface reaction of a gas molecule “A”, the

fraction of surface covered by A is?

KP

1+ KP +KI

Where “I” is the pressure of the inhibitor and “K” is the equilibrium constant.

6. Write the rate expressions for chemisorptions and desorption processes?

The rate expression for bimolecular surface reaction of A and B gas molecules assuming that

Langmuir-Hinshelwood mechanism are following

The rate expression when two molecules adsorbed on the surface of the catalyst is

kKAKB[A][B]

(1+KA[A]+KB[B])2

6. What is non-ideal solution? Explain activity, activity coefficients and chemical potential?

An ideal solution is a model system in which every component has its chemical potential.

i = i + RT ln xi; where i is the chemical potential of the pure substance and xi is the

mole fraction of the substance. A non-ideal solution is the one that is not an ideal solution and

which does obey neither Raoult’s law nor Henry’s law. For non-ideal solution, chemical

potential is given by i = i + RT ln ai; where ai is the activity of ith

ion. Ai replaces xi in the

expression for i in a non-ideal solution.

ai=xi for ideal solution; when ai=0: i = i

i- iId

= (i + RT ln ai) – (i + RT ln xi) = RT ln (ai/xi)

(ai/xi) is the measure of departure from ideal behaviour and it is known as activity coefficient

(i)

For non-ideal solutions: i = i + RT ln ixi; if i= 1 for ideal solution. If only single

component is present in the system, then xi=1 and i is also one; then i = i + RT ln i. It

becomes i = i

= G/ T, P, ij; It is a measure of reactivity of a component in the solution.

Thermodynamic function of a substance in a system that is the partial differential of the Gibbs

functions of the system with respect to the number of moles of the substance. Symbol μ

8.